The placenta is formed by the foetus and starts its development around 35 days (34-37) when cells from the developing foetal unit migrate into the endometrium (uterine lining) of the mare to form what are called endometrial cups. The placenta continues to develop and expand throughout the uterus until it is physically in contact with the entire endometrium. The foal develops predominantly in one side of the uterus or the other, depending on whether it implanted at the junction between the horn and the body of the uterus on the right or the left side.
The placenta itself consists of two tissues – the chorion and the allantois – that fuse together, with the allantois being the surface on the foetal side, and the chorion, the surface on the maternal side. It is the chorionic layer of the placenta that has the bright red velvet appearance due to the small, finger-like projections of blood vessels. These placental blood vessels interlace with the endometrium vessels, forming a tight bond that persists until the foal is born.
It is this interaction of the placental structure with the endometrium that provides oxygen, nutritional support, and waste removal to the foal during its term in the uterus. The placenta/uterus apposition forms a unique functional unit that allows oxygen and nutrients to pass from the maternal blood circulation into the blood vessels of the placenta, and into the foal via the umbilical cord, and the return of carbon dioxide and waste products from the foal back through the placenta and into the mare’s blood system to then be expelled from her body.
As the mare goes through the second stage of labour and the foal is delivered, the umbilical cord ruptures causing a drop in blood pressure, which, together with the rhythmic contractions of the uterine muscle, brings about the final stage of labour, the release of the placenta and its expulsion from the uterus. In the typical foaling scenario, the placenta is expelled within three hours of the foal being delivered, and more commonly within one hour. The placenta is usually expelled inside out, so that the white side of the allantois is the side we see, with the red chorion on the inside.
Foetal membranes that are not released within three hours are generally referred to as delayed foetal membranes and should alert the owner that retention of the membranes is a strong possibility and plans for veterinary advice or intervention should be sought. Membranes that have not been expelled by 6-8 hours are referred to as an RFM and require veterinary assistance as soon as possible. Typically, it is the non-pregnant horn of the placenta that is the last part of the membranes to detach from the uterus.